Battery cells are used in a wide variety of electronic and electrical devices. Very often, individual battery cells are connected electrically in series to form battery packs having higher total voltage and higher total energy capacity than individual battery cells. One consequence of connecting battery cells in series is that all of the battery cells will experience the same electrical current. Therefore, the number of ampere-hours added or subtracted from every battery cell's charge level will be the same. However, individual battery cells may have different charge capacity (in ampere hours). This is particularly true as the battery pack ages, because over time individual battery cells degrade: battery cell resistances tend to increase and battery cell charge capacities tend to decrease. If this process occurs at different rates in different battery cells in a battery pack, then at some point in time one or more battery cells may limit the battery pack total energy capacity. A battery cell having lower charge capacity is more quickly charged to the maximum design voltage, and is more quickly discharged to the minimum design voltage. Therefore, either not all battery cells will be fully charged when the battery cell having lower charge capacity is charged, or not all battery cells will be fully discharged when the battery cell having lower charge capacity is discharged. Not all of the possible energy capacity of the battery pack will be realized.
At some point, a battery pack may be capable of higher total energy capacity if one or more of the battery cells having low charge capacity are removed from the battery pack so that they no longer limit the range of charge of the other battery cells. In some applications, these battery cells might be replaced with new battery cells. In other applications, that might not be practical, so the battery controller could then allow those battery cells to be abandoned—the lower voltage of these particular battery cells would no longer be restricted by the controller to the lower design voltage. This “undercharge condition” for some battery cell electro-chemistries causes the battery cell voltage to collapse to zero and the battery cell charge capacity to collapse to zero, effectively eliminating that battery cell from the battery pack without physically removing it. This is a safe operating condition, and can result in higher overall battery pack energy capacity.
While abandoning a battery cell in this manner can result in higher overall battery pack energy capacity, it will also place greater stresses on the remaining battery cells in the battery pack, since they must provide greater power levels per battery cell than they did before. Therefore, in some applications it may be desirable to optimize the battery pack total energy capacity. Battery pack total energy capacity is one possible battery pack total energy metric. In other applications it may be desirable to optimize a different battery pack total energy metric that takes into account the total energy capacity and stress factors on remaining battery cells when some battery cells are abandoned.
Accordingly, there is a need for a method for efficiently determining a battery pack configuration based on present battery cell charge capacities that maximizes a battery pack total energy metric. The embodiments disclosed herein perform this task.